Although initially conceived for off-road driving, it has been found that four wheel drives can be advantageous for on-road driving too, since they are less susceptible to problems like speed-dependent oversteer or understeer, which are common with front or rear wheel driven vehicles. However, the advantage of improved steerability of four wheel driven vehicles tends to be outweighed by their fuel consumption, which is generally higher than that of a similar two-wheel driven vehicle, due to the increased mass of rotating components and to increased friction losses. A driver who is in want of the improved steerability of the four wheel drive is thus continuously penalized by high fuel consumption, whereas a driver who practices an economical driving style has to take the risk of sub-optimal steerability.
At least one object of the present invention is to provide a motor vehicle, a method for controlling a clutch system in a motor vehicle and a data processor program product which allow combining the improved steerability of the four wheel drive with the economic operation of front or rear wheel drives. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
This at least one object, and other objects, desirable features, and characteristics, is achieved, according to a first embodiment of the invention, by providing a motor vehicle having an engine, front and rear wheels and a clutch system for selectively distributing engine torque to the front and rear wheels with a controller for judging in real time the state of motion of the vehicle and controlling the torque distribution by the clutch system according to the judged state.
Based on this judgment, the distribution of the torque to the front and rear axles is continuously adapted to current driving conditions, and the use of the four wheel drive may be restricted to situations where it is effectively helpful.
Control of the torque distribution is preferably carried out by the controller selecting one from a plurality of pre-determined discrete distribution ratios. In this way, if there are other parameters of the vehicle chassis besides the torque distribution which are also adapted according to the judged state of motion, the number of possible combinations of values of the torque distribution and of the other parameter(s) is finite, and each combination can be checked for operational safety, and selection of an unsafe parameter combination can be prevented easily.
The controller should be adapted to distinguish between at least a high acceleration state and a low acceleration state.
In the high acceleration state, being more susceptible to oversteer or understeer, the torque transmitted by the clutch system should be distributed more evenly between front and rear wheels (i.e., should be more like that of a four wheel drive than in a low acceleration state).
In order to prevent the wheels of the vehicle from slipping, in particular in case of high longitudinal acceleration, the controller is preferably adapted to apply a higher portion of the torque to the rear wheels in a high acceleration state than in a low acceleration state. 
Generally speaking, a high acceleration state may be judged to exist if at least one of the following conditions is met:                the lateral acceleration of the vehicle exceeds a first predetermined threshold;        the longitudinal acceleration of the vehicle exceeds a second predetermined threshold;        the side-slip velocity of the vehicle exceeds a third predetermined threshold;        the rate of change of a steering wheel angle exceeds a fourth predetermined threshold;        the rate of change of the accelerated pedal or the throttle position or the fuel injection rate exceeds a fifth predetermined threshold; and/or        the vehicle is in an engine braking mode.        
At least in the last three cases, the amount of acceleration at the instant of judging may still be small, but it is clear to the skilled person that a substantial acceleration is likely to occur soon if the steering wheel is turned quickly or the accelerator pedal is depressed quickly. In the engine braking mode, acceleration with a negative sign has to be expected.
Further, the controller is preferably adapted to distinguish between a high speed state and a low speed state and to distribute the torque transmitted by the clutch system between front and rear wheels more evenly in the low speed state than in the high speed state. By applying driving torque to front and rear wheels while the vehicle is moving slowly or standing still, launching the vehicle on slippery ground is facilitated.
According to a second embodiment of the invention, the at least object, and other objects, desirable features, and characteristics, is achieved by a method for controlling a clutch system in a motor vehicle for selectively distributing engine torque to front and rear wheels of the vehicle, in which the state of motion of the vehicle is judged in real time, and the torque distribution by said clutch system is controlled according to the judged state. 
The embodiments of the invention may be embodied by a data processor program product comprising a data carrier in which program instructions for enabling a data processor to form the controller described above or to carry out the method said out above are recorded in machine readable form.